1. Field of the Invention
The invention relates generally to navigation satellite receiver systems and more specifically to manufacturing and analysis fixtures used to test and verify the proper operation and initialization of real-time kinematic receiver systems.
2. Description of the Prior Art
Centimeter-accurate global positioning system (GPS) navigation depends on being able to resolve individual cycles of carrier phase from an orbiting satellite to a navigation receiver. The many cycles of phase that could be in the neighborhood of a code-based position solution create an ambiguity that is difficult, but not impossible to resolve. The integer number of cycles to each of several satellites simultaneously indicates the correct unique integer-ambiguity solution.
Real-time kinematic (RTK) surveying is a valuable branch in the science of GPS positioning. RTK has substantially improved surveying productivity in the field. RTK eliminates the time consuming post-processing of satellite data that had been an inescapable part of conventional kinematic and static GPS surveying. Quality assurance indicators are produced in real-time that guarantee the results will be good before vacating a site. In the past, cycle slips, especially at the reference GPS receiver, prevented post-processing the kinematic data and such problems were latent and ruinous. With RTK, this and other blunders in field procedures are detectable and thus costly re-surveys can be avoided.
Although RTK systems, such as the SITE SURVEYOR.TM. from Trimble Navigation (Sunnyvale, Calif.), have been commercially available for some time, such products require static initialization for carrier integer ambiguity resolution. The maximum benefits of RTK are only realized when such initialization processes are independent of system motion, are fully automatic and are transparent to the user/surveyor.
The performance of RTK systems is often judged by the reliability of initialization and the time it takes a receiver to initialize, both of which are interrelated. The time it takes a receiver to initialize is defined here as the time needed to produce the first centimeter-level accurate output, e.g., after a complete loss of lock on all satellites. Test conditions require four, preferably five or more satellites to be visible, in order to rely on highly efficient integer search strategies. The accuracy of the centimeter-level output of a RTK system is also a key element of the system performance.
Up until a few years ago, commercial RTK systems for land surveying were simply not available. However, GPS systems for navigation and positioning were already a well-established industry standard for a variety of geodetic survey applications. Surveyors using GPS systems relied on traditional post-processing with data collection times that could range up to an hour. A technique called FAST STATIC.TM. data collection reduced this to a few minutes. Post-processed kinematic was effective, but was risky without good satellite visibility, especially without knowledge of satellite tracking at the base receiver.
The ability to perform surveys in real-time has many benefits across a variety of applications. Real-time communications between the reference and multiple rover stations provides integrity checking. Users are able to navigate to survey marks very accurately. But static RTK systems require the user to suffer an initialization procedure while the receiver remains stationary when first used in the field.
During initialization, the conventional GPS surveyors require occupation of a known survey mark or the location of two antennas approximately at the same place using an initializer plate. These constrain the field procedure, and can cause problems when the satellite signals become obstructed, e.g., when a user passes under a bridge. In such a case the user would be forced to return to a known point, or reinitialize the survey in some other manner.
Fully automatic ambiguity resolution (FAAR), as commercially developed by Trimble Navigation, avoids having to initialize from a known mark. A stationary base unit provides reference signals to a "rover" unit that moves about to conduct a survey. There is no constraint on the rover during initialization, it may be stationary or moving. This process has two performance parameters associated with it, the initialization reliability and the time to. initialize. Both initialization time and initialization reliability are key criteria for a commercial user of a real-time kinematic system.